Rename DUMMY_MEDIA_ID and related cleanup

Issue: #7565
PiperOrigin-RevId: 353616594

library/core/src/main/java/com/google/android/exoplayer2/source/SinglePeriodTimeline.java

@@ -31,10 +31,7 @@ public final class SinglePeriodTimeline extends Timeline {
 
   private static final Object UID = new Object();
   private static final MediaItem MEDIA_ITEM =
-      new MediaItem.Builder()
-          .setMediaId("com.google.android.exoplayer2.source.SinglePeriodTimeline")
-          .setUri(Uri.EMPTY)
-          .build();
+      new MediaItem.Builder().setMediaId("SinglePeriodTimeline").setUri(Uri.EMPTY).build();
 
   private final long presentationStartTimeMs;
   private final long windowStartTimeMs;

library/dash/src/main/java/com/google/android/exoplayer2/source/dash/DashMediaSource.java

@@ -295,7 +295,7 @@ public final class DashMediaSource extends BaseMediaSource {
           manifest,
           new MediaItem.Builder()
               .setUri(Uri.EMPTY)
-              .setMediaId(DUMMY_MEDIA_ID)
+              .setMediaId(DEFAULT_MEDIA_ID)
               .setMimeType(MimeTypes.APPLICATION_MPD)
               .setStreamKeys(streamKeys)
               .setTag(tag)
@@ -427,8 +427,7 @@ public final class DashMediaSource extends BaseMediaSource {
   /** @deprecated Use {@link #DEFAULT_FALLBACK_TARGET_LIVE_OFFSET_MS} instead. */
   @Deprecated public static final long DEFAULT_LIVE_PRESENTATION_DELAY_MS = 30_000;
   /** The media id used by media items of dash media sources without a manifest URI. */
-  public static final String DUMMY_MEDIA_ID =
-      "com.google.android.exoplayer2.source.dash.DashMediaSource";
+  public static final String DEFAULT_MEDIA_ID = "DashMediaSource";
 
   /**
    * The interval in milliseconds between invocations of {@link